1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor elements and particularly for forming a polycrystal silicon film on the capacitor electrode surface.
2. Description of the Related Art
Due to the needs for more highly integrated semiconductor devices, further reduction in the cell size is being sought. Particularly in the field of the Dynamic Random Access Memory (DRAM) for which one bit is composed of one transistor and one capacitor, if the cell size is reduced, the electrode area of the capacitor is decreased, hence the capacity value is decreased. As a result, problems such as lowered data hold time and incapability in preventing memory loss caused by an alpha ray will occur.
One method to solve this problem is to use a capacitor with a three-dimensional cylinder structure or a fin structure. However, this method has techincal limitations.
As other methods, there is a method to increase the capacity value by using tantal oxide (Ta.sub.2 O.sub.5) with a high induction rate or barium strontium titanate (Ba.sub.(x) Sr.sub.(1-x) TiO.sub.3) with a strong induction film. However, this method has not been made fit for practical use.
As another notable method, there is a method called the HSG process that increases the capacity value by making the capacitor surface uneven in order to increase the surface area.
FIG. 1 roughly shows how work progresses in the HSG process. As shown in FIG. 1(a), the amorphous silicon film (1) that is the capacitor understructure electrode is formed on the intercalation layer (3) formed on the silicon substrate (8). The semiconductor substrate (8) and the amorphous silicon film (1) are linked by polycrystal silicon (9). Also, naturally formed oxide film (2) adheres to the amorphous silicon film (1). After the naturally formed oxide film (2) is washed off during pre-processing, the clean surface of the amorphous silicon film (1) is exposed. At this point, hydrogen atoms (5) are bonded to the dangling bonds on the surface of the amorphous silicon film (1) (FIG. 1(b)). This hydrogen (5) is desorbed by being heated at a processing temperature of approximately 560.degree. C. and the surface of the amorphous silicon film (1) becomes activated (FIG. 1(c)). In an atmosphere of monosilane (SiH4) gas, the mixed-phase active layer of amorphous-polycrystal silicon (6) is then selectively formed on the activated surface area by surface reaction (FIG. 1(d)). At this point, if it is annealed at a temperature of approximately 560.degree. C. for a predetermined time period, the mixed-phase active layer amorphous migrates with polycrystal silicon on the surface as a nucleus, crystallizes into polycrystal silicon and the polycrystal silicon grain (7) grows. As a result, highly crystalline silicon grains (HSG) (7) are formed on the amorphous silicon electrode, resulting in a rough surface (FIG. 1(e)).
Normally, phosphorus (P) is doped on the amorphous silicon electrode surface. For methods for doping phosphorus, there are such methods as Chemical Vapor Deposition (CVD equipment) and Surface-reaction thin film Formation. The former is a method for doping phosphorus at the same time the amorphous silicon film is formed. The latter is a method for selectively growing phosphorus-doped amorphous-polycrystal silicon mixed-phase active layer on the active surface of amorphous silicon.